The personalized editing therapy developed at record speed for the American newborn required preliminary studies, favorable circumstances, and a heroic collective effort.

After 307 days, little KJ Muldoon was discharged from the Children’s Hospital of Philadelphia (CHOP), wearing a tiny graduation outfit complete with a blue gown and cap. Born ten months ago with a severe metabolic disorder, the baby received a genome editing therapy developed exclusively for him, and his remarkable progress has been hailed by many as the dawn of a new era in precision medicine. A month after the publication of his case in the New England Journal of Medicine, we take a closer look at how researchers managed to develop the treatment in just six months—and whether this breakthrough could be replicated for other rare disease patients in need of life-saving therapies.